The Cambrian is the earliest period in whose
rocks are found numerous large, distinctly fossilizable multicellular
organisms. This sudden appearance of hard body fossils is referred
to as the Cambrian
explosion. Despite the long recognition of its distinction from
younger Ordovician rocks
and older Precambrian
rocks it was not until 1994 that this time period was
internationally ratified. The base of the Cambrian is defined on a
complex assemblage of trace
fossils known as the Trichophycus
pedum assemblage. This assemblage is distinct from anything in
the Precambrian as it has ecologically tiered vertical burrows
which are absent from the Precambrian.

Cambrian subdivisions

The Cambrian period follows the
Ediacaran
and is followed by the Ordovician
period. The Cambrian is divided into three epochs
— the Early
Cambrian (Caerfai or Waucoban), Middle
Cambrian (St Davids or Albertian) and Furongian (also
known as Late Cambrian, Merioneth or Croixan). Rocks of these
epochs are referred to as belonging to the Lower, Middle, or Upper
Cambrian.

Each of the epochs are divided into several
stages. Only one, the Paibian, has been
recognized by the
International Commission on Stratigraphy, and others are still
unnamed. However, the Cambrian is divided into several regional
faunal
stages of which the Russian-Kazakhian system is most used in
international parlance:

Cambrian dating

The time range for the Cambrian has
classically been thought to have been from about 500 mya to about
570 mya. The lower boundary of the Cambrian was traditionally set
at the earliest appearance of early arthropods known as trilobites and also unusual
forms known as archeocyathids
(literally 'ancient cup') that are thought to be the earliest
sponges and also the first non-microbial reef builders.

The end of the period was eventually set at a
fairly definite faunal change now identified as an extinction
event. Fossil discoveries
and radiometric
dating in the last quarter of the 20th century have called
these dates into question. Date inconsistencies as large as 20
Ma are
common between authors. Framing dates of ca. () 545 to 490 mya were
proposed by the International Subcommission on Global Stratigraphy
as recently as 2002.

A radiometric date from New
Brunswick puts the end of the first stage of the Cambrian
around 511 mya. This leaves 21 Ma for the other two stages of the
Cambrian.

A more precise date of 542 ± 0.3 mya for the
extinction event at the beginning of the Cambrian has recently been
submitted. The rationale for this precise dating is interesting in
itself as an example of paleologicaldeductive
reasoning. Exactly at the Cambrian boundary there is a marked
fall in the abundance of carbon-13, a
"reverse spike" that paleontologists call an
excursion. It is so widespread that it is the best indicator of the
position of the Precambrian-Cambrian boundary in stratigraphic sequences of
roughly this age. One of the places that this well-established
carbon-13 excursion occurs is in Oman. Amthor (2003)
describes evidence from Oman that indicates the carbon-isotope excursion relates to a
mass extinction: the disappearance of distinctive fossils from the
Precambrian
coincides exactly with the carbon-13 anomaly. Fortunately, in the
Oman sequence, so too does a volcanic ash
horizon from which zircons provide a very precise
age of 542 ± 0.3 Ma (calculated on the decay rate of uranium to lead). This new and precise date
tallies with the less precise dates for the carbon-13 anomaly,
derived from sequences in Siberia and
Namibia. It
is presented here as likely to become accepted as the definitive
age for the start of the Phanerozoic eon, and thus the start of the
Paleozoic era and the Cambrian period.

Cambrian fauna

Of those modern animal phyla that fossilize
easily, all save the bryozoans have claimed
representatives in the Cambrian. Many extinct phyla
and odd animals that have unclear relationships to other animals
also appear in the Cambrian. The apparent "sudden" appearance of
very diverse faunas over a period of no more than a few tens of
millions of years is referred to as the "Cambrian
Explosion". Also, the first possible tracks on land, such as
Protichnites
and Climactichnites,
dating to about 530 mya and found in Ontario, Canada, and northern
United States, appeared at this time. The conodonts, small predatory
primitive chordates known from their fossilised teeth, also
appeared during the Furongian epoch of the Cambrian period. The
conodonts thrived throughout the Paleozoic and the early Mesozoic
until they completely disappeared during the Late Triassic period
when the first mammals were evolving.

The best studied sites where the soft parts of
organisms have fossilized are in the Burgess
shale of British
Columbia. They represent strata from the Middle Cambrian and
provide us with a wealth of information on early animal diversity.
Similar faunas have subsequently been found in a number of other
places — most importantly in very early Cambrian shales in the
People's Republic of China's Yunnan Province (see
Maotianshan
shales). Fairly extensive Precambrian Ediacaran
faunas have been identified in the past 50 years, but their
relationships to Cambrian forms are quite obscure.

Cambrian flora

Generally it is accepted that there were no
land
plants at this time although molecular dating suggests that
land plant ancestors diverged from the algae much earlier, in the
Neoproterozoic
about 700 ma, and that
fungi diverged from the
animals about 1 billion years ago. The land at this time was
barren, mostly desert and
badlands.